This is it - the game which started it all. The game which revolutionized Adventure games by finally throwing out the text parser completely. The game which put Lucasfilm's games division into the limelight, where they stayed for almost ten years. And even when they stopped producing these great Adventure games, nobody else was there to take their place. But I'm disgressing.
Maniac Mansion hadn't been Lucasfilm's first Adventure game. Only a year earlier, they had released Labyrinth, a conversion of their then popular movie, still using a traditional interface, although already with a very similar graphical style. MM was created using the newly developed SCUMM system (Script Creation Utility for Maniac Mansion) which would, in its basics, remain unchanged until 1993.
From today's point of view, it's not the interface which makes the game still shine, though, but the hilariously funny story: In the best tradition of classic mad scientists, Dr Fred Edison has kidnapped a teenage girl called Sandy to perform brain-related experiments on her in his mansion. Sandy's boyfriend Dave, accompanied by two of his friends, has to rescue her, so they enter the Edison mansion and soon meet with the other inhabitants: nurse Edna, mad Ed Edison, Fred's militarily (does that word even exist?) obsessed son and... two walking and talking tentacles, of whom one turns out to be quite nice, in fact.
Depending on which two out of Dave's seven available friends are chosen by the player, puzzles and the resulting ending of the game vary a bit to enhance replayability. If a character is described as a talented writer, you can be sure this ability will be used in the game, for example. Searching for walkthroughs of the game on the net these days, you'll notice that all of them deal with the same one or two character combinations, so there's still a lot of apparantely untrodden ground to discover.
Maniac Mansion also has the typical Lucasfilm / Lucas Arts in-jokes, many of which even have their roots right here. Chuck the plant makes its first appearance in the Edison library, in the kitchen, you can pick up the legendary chainsaw (there are still people actively looking for the fuel I hear), and at one point, an alien from the earlier Lucasfilm game Rescue on Fractalus has a cameo.
The humor and the puzzles are indeed the timeless part of the game. Graphically, it's ok and nice, but hardly worth raving about. Same for the sound. And even the SCUMM interface (whose first incarnation in fact only appears in the C64 versions of Maniac Mansion and Zak Mc Kracken - other ports already used a slightly tweaked version) has its glitches. For example, it's hardly understandable why the verb 'What is' is included at all. It makes the cursor sensitive to objects, showing their names when it touches them. What's the point of not having this always on by default? On a positive note, the verb 'Use' isn't yet as omnipotent as it became later, because there are still quite a few more specific variations of it available.
No matter whether you look at it from a historical point of view or just to find a great Adventure game - Maniac Mansion always fits. Its importance in the history of computer games is undeniable, and so is the quality of the gameplay and story. What are you waiting for? Enter the maniac mansion once again!